Pulmonary arterial hypertension (PAH) is a rare, progressive, and life-threatening disease. In many patients, the course of PAH is a steady deterioration and reduced life expectancy.

Sildenafil was approved by the European Commission for the treatment of PAH in pediatric patients in May 2011, making it the first agent to be approved for the treatment of children with PAH. The approval was based on the largest placebo-controlled study to be conducted in this population. The recommended dose in pediatric patients aged 1 year to 17 years old is 10 mg TID in patients ≤ 20 kg and 20 mg TID for patients > 20 kg. Higher doses are not recommended in pediatrics patients.

This study is an open-label, multi-center study to investigate safety, efficacy and pharmacokinetics of sildenafil citrate in Japanese pediatric patients with PAH.

Change from Baseline in World Health Organization (WHO) Functional Class in Participants with Pulmonary Arterial Hypertension (PAH) at each visit [ Time Frame: From Baseline to End of treatment ] [ Designated as safety issue: No ]

Subjects who have symptomatic pulmonary arterial hypertension due to one of the following conditions:

Idiopathic pulmonary arterial hypertension; or

Heritable pulmonary arterial hypertension; or

Pulmonary arterial hypertension associated with congenital systemic-to-pulmonary shunts. If the defect(s) is repaired, the subject's condition should be stabilized hemodynamically; or

Pulmonary arterial hypertension associated with d-transposition of the great arteries repaired within the first 30 days of life; or

Pulmonary arterial hypertension in subjects who have undergone surgical repair of other congenital heart lesions and the condition should be stabilized hemodynamically and do not have clinically significant residual left-sided heart disease.

Subjects with a mean pulmonary artery pressure ≥25 mmHg at rest, PCWP ≤15 mmHg, and PVRI ≥3 Wood units x m2. If PCWP is not available, then mean LA pressure ≤15 mmHg or LVEDP ≤15 mmHg in the absence of left atrial obstruction.

Subjects receiving chronic treatment with off-label sildenafil within 30 days of Day 1 are excluded. Subjects receiving an endothelin antagonist，PED5 inhibitor or, prostacyclin/prostacyclin analogue within 30 days of randomization are excluded except for beraprost.

Pregnant females; breastfeeding females; males and females of childbearing potential not using highly effective contraception or not agreeing to continue highly effective contraception for at least 28 days after last dose of investigational product.

Current or past illicit drug use or alcoholism excepting if abstinence can be documented for ≥1 year.

Participation in another clinical trial of an investigational drug or device (including placebo) within 30 days of screening for entry into the present study.

Other severe acute or chronic medical or psychiatric condition or laboratory abnormality that may increase the risk associated with study participation or investigational product administration or may interfere with the interpretation of study results and, in the judgment of the investigator, would make the subject inappropriate for entry into this study.

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Please refer to this study by its ClinicalTrials.gov identifier: NCT01642407